1. Field of the Invention.
The present invention relates generally to optical disk drives. In particular, the present invention is a drive motor and spindle assembly for centering an optical disk on an axis of rotation within an optical disk drive.
2. Description of the Prior Art.
The demand for mass data storage continues to increase with expanding data processing operations and personal computer use. Optical data storage systems are becoming an increasingly popular means for meeting this need. With systems of this type, extremely large quantities of data can be stored and quickly accessed at relatively low cost.
Optical data storage systems include an optical disk for storing information in digital form. Optical disk drives utilize a circular rotating disk with one or more recording surfaces. Data is recorded on spiral or concentrically positioned tracks centered about the disk's axis of rotation. The disk is loaded into an optical disk drive which includes mechanical, electrical and optical systems required to write data to and read data from the optical disk. A feature of those systems is a recording head which is moved radially across the disk to access the track that data is to be written to or read from.
Magnetic data storage systems which use magnetic disks are well known. Magnetic disks have a relatively low density of concentric recording tracks. Because of the relatively low density of recording tracks on magnetic disks, disk alignment with respect to the rotational axis of the drive unit requires only moderate precision to ensure acceptable alignment of the recording head over the desired recording track.
The optical data disks used in conjunction with optical disk drives have a very high density of recording tracks. Higher performance tracking systems are therefore required to keep the optical read/write head centered on the desired servo track. Track runout, the radial displacement of the servo tracks while the disk is rotated, must be kept to a minimum. One method for reducing runout is to accurately center the disk on the rotational axis of the drive unit.
A variety of spindle assemblies for centering optical disks on their drive motors are known. One spindle assembly includes a cup having a lip which forms a receiving platform for the optical disk. The cup contains a magnet which holds the disk in contact with the lip. A vertically movable and conically shaped centering hub having inclined side surfaces is positioned within the cup and is concentric therewith. The centering hub is biased upward by a helical compression spring positioned around the drive shaft of the motor. The optical disk used with this mechanism includes a mounting hub concentrically positioned about its mounting hole. When the disk is placed on the spindle assembly its position is determined by the contact of its mounting hub with the centering hub. Magnetic attraction between the magnet in the cup and the mounting hub of the disk cause the disk and centering hub to move downward until the disk makes contact with the lip.
The helical compression spring in the above-described spindle assembly requires relatively high tolerances so that the centering hub deflects accurately and keeps the disk centered on the rotational axis of the drive motor. The higher the tolerances to which this and other parts must be manufactured, the higher their cost. It is expensive and time consuming to accurately center the mounting hub on the disk during its manufacture. Clamping the mounting hub on the disk also compresses the disk and increases the likelihood of birefringence which results in optical distortion.
Another spindle assembly includes a cup-shaped hub concentrically mounted about the shaft of a drive motor, and a conically tipped centering pin extending from the hub coaxial with the shaft. Optical disks used with this spindle assembly have a mounting hub with a pin-receiving hole concentrically clamped to the disk. A ring magnet mounted on the spindle hub around the centering pin holds the mounting hub and disk on the spindle assembly. This spindle assembly suffers from some of the same problems as that discussed above. Tolerances of the spindle centering pin and hole in the disk hub must be relatively high to keep runout to acceptable levels. The centering pin and disk hub hole are also subject to wear which can increase runout.
The Elliott U.S. Pat. No. 4,326,284 discloses a video disk player with a record centering spindle. The spindle is cone shaped and disposed on a shaft of the drive motor. The spindle is biased toward a raised position by a bias spring in the spindle shaft. Other spindle assemblies are disclosed in the following U.S. patents: Konshak U.S. Pat. No. 4,736,356, Tsuji U.S. Pat. No. 4,517,617, Camerik U.S. Pat. No. 4,135,721, Jansen et al. U.S. Pat. No. 4,545,046 and Caletti U.S. Pat. No. 3,938,192. None of them, however, appear to provide a centering mechanism that is free of all the problems discussed above.
It is evident that there is a continuing need for improved optical disk spindle assemblies. An easy to manufacture and inexpensive drive motor and spindle assembly which can accurately center a disk and has a relatively low sensitivity to wear is needed. The drive motor and spindle assembly should be small enough for use in conjunction with half-height drives which fit standard personal computers.